1. Field of the Invention
This invention relates to steel cords for the reinforcement of rubber articles used as a reinforcing member for heavy duty pneumatic radial tires, conveyor belt and the like. It also relates to a method of producing such steel cords.
2. Description of the Related Art
In rubber articles including pneumatic tires, steel cords are widely used as a reinforcing member. For example, steel cords having 3+9+15 twisting structure obtained by varying a twisting pitch of steel filaments having the same filament diameter every layer are widely used as the steel cord used for heavy duty tires in order to make large a cord breaking load per diameter of the cord and provide good fatigue resistance. However, the steel cord of such a structure has no space for penetrating rubber into the inside of the cord as shown in FIG. 4, so that if the cord contacts water, corrosion of the cord is caused by water penetrating into the spaces inside the cord containing no rubber.
Furthermore, there is known a steel cord of compact twisting structure in which 24 steel filaments are simultaneously twisted around 2 filaments as a core as shown in FIG. 3. Such a steel cord has also a problem in corrosion resistance because a space for penetrating rubber is not existent inside of the cord.
In order to solve such a corrosion problem, JP-B-62-18678 and JP-A-4-257382 propose steel cords capable of improving rubber penetrability into the inside of the cord by decreasing the number of steel filaments used in the sheath layer and forming a space between adjoining steel filaments in the sheath layer, respectively.
Furthermore, JP-U-64-30398 proposes that the corrosion problem can be solved and the working cost in the twisting step can be reduced by aligning the steel filaments in the core layer of the steel cord of two-layer or three-layer twisting structure in parallel with each other.
The steel cord proposed in JP-B-62-18678 has a three-layer twisting structure consisting of a core formed by twisting 2-4 steel filaments, a middle sheath layer and an outer sheath layer, in which a space between the steel filaments in each of the middle and outer sheath layers is specified to a given value. In the example of this publication, there is disclosed, for instance, a steel cord of 2+7+12.times.0.22 structure in which a space ratio between the filaments in the middle sheath layer is 22.6% and a space ratio between the filaments in the outer sheath layer is 21.0%. Furthermore, there are described such twisting pitch and twisting direction that the core is S lay at a twisting pitch of 5 mm, the middle sheath layer is S lay at a twisting pitch of 10 mm and the outer sheath layer is Z lay at a twisting pitch of 15 mm.
In this steel cord, however, the diameter of the steel filament constituting the core is the same as in the sheath, so that the section of the steel cord becomes too elliptical. Therefore, when such a steel cord is used in a cross belt layer of a pneumatic radial tire, there is caused a problem that a torsional deformation is apt to be created in the steel cord during running of the tire to bring about fatigue breakage.
In JP-U-64-30398 is disclosed a steel cord of two-layer or three-layer twisting structure having a core formed by aligning 2-3 steel filaments in parallel with each other. In the example of this publication, the number of the steel filaments in the core is 3 or more and a space not penetrating rubber is formed in the center of the core. When such a steel cord is used in the belt layer of the tire, it is anticipated that there is caused a problem of penetrating water from a cut portion formed in the running on bad road into the inside of the cord to corrode the cord. Further, since the diameter of the steel filament constituting the core is the same as in the sheath, the fatigue breakage is apt to be caused likewise the aforementioned case.
In JP-A-4-257382 is proposed a steel cord in which the steel filaments in the core, middle sheath layer and outer sheath layer are twisted in the same direction and the twisting pitch is the same in the core and outer sheath layer and the twisting pitch in the middle sheath layer is shorter than those of the core and outer sheath layer. In the example of this publication, there is proposed, for instance, a steel cord of 2+5+11.times.0.22 structure in which the twisting pitch (twisting direction) is 18 (S)/ 9 (S)/18 (S).
Such a steel cord has also a problem of fatigue breakage because the diameter of the steel filament in the core is the same as in the sheath.
In order to improve rubber penetrability, when the number of steel filaments in the sheath is reduced excessively from the compact structure, the tenacity of the cord lowers to impair the function as a reinforcing member. Also, the arrangement of the steel filaments in the sheath is disturbed to make the space between the adjoining steel filaments ununiform and hence the penetration of rubber is rather damaged.